lol - yep, 3d modelling and 3d modelling for 3d printing are 2 totally different animals.

Two ways to do this.
1) enable supports in the slicer and it will print supports for any areas of the model that won't ordinarily print.
2) design it specifically for 3d printing.

The way I suggest is to slice the model and then go through the preview slowly. Anywhere you get material appearing in mid air will be a print failure.
You can build supports into your model yourself, or let the slicer do it.

what slicer are you using ?